Packaging materials, such as paperboard, paper or fabric are frequently utilized to package grease, fat or oil-containing materials. For example, human and animal foodstuffs commonly include a significant amount of grease, fat or oil. During filling, transport or storage, the grease, fat or oil from the foodstuff can migrate from the food product to the packaging material, thereby resulting in unsightly stains on the outer surface of the packaging material. Also, grease, fat or oil-containing materials may come into contact with the outer surface of the packaging material to cause staining. While this migration of grease, fat or oil does not normally effect the structural integrity of the packaging material, the consumer generally dislikes the stained container and may be less likely to purchase a product because of such staining.
One solution to the problem of grease, fat or oil staining on a packaging material is to place the material to be packaged within an separate impermeable container within the packaging material itself such as, for example, a plastic bag. While such a method normally prevents grease, fat or oil staining in the packaging material prior to use by the consumer of the packaged material, this method has less effectiveness when the package has been opened and returned to the consumer's shelf for storage. For example, grease, fat or oil-containing crackers may be placed in a plastic bag within a packaging material. Grease, fat or oil will be contained within the bag as long as the bag remains closed. However, once opened, it is quite common for a consumer to drop one or more crackers between the bag and the inner surface of the packaging material when removing the crackers for use. If there is no bather material between the outer surface of the plastic bag, it is likely that grease, fat or oil will migrate from the cracker into and through the packaging material, thereby resulting in an unsightly stain on the outer surface of the packaging material. Stains on packaging material may also be caused by filling line debris or puncturing of the plastic bag.
Stains on the packaging material may also be caused by contact by a grease, fat or oil-containing material with the outer surface of the packaging material. Such stains may be from contact of the package with a grease, fat or oil stain on another package or by contact with grease, fat or oil-containing substances that may occur during filling, transport or storage.
Furthermore, it will be understood that, irrespective of the origin of a grease, fat or oil stain, extended times and elevated temperatures will normally exacerbate the appearance of such stains.
Whether or not a plastic bag is placed within the packaging material, in the past, it has been common to treat the packaging material with a barrier material effective to reduce the possibility that grease, fat or oil will migrate into and through the packaging material to cause a visible stain on the outer surface of the packaging material. The most common barrier materials from the standpoint of cost and effectiveness have been those of the fluorochemical variety i.e., Scotchguard® products, made by the 3M Company of Minneapolis, Minn. These materials have generally been applied to the packaging material prior to formation into, for example, a box. However, due to environmental and health concerns, 3M's fluorochemical barrier products have recently been removed entirely from the market. While other barrier materials exist, many otherwise effective methods are either not acceptable as the sole method of protection for use in certain applications or are applied from solvent-based systems. Moreover, for some of the more useful methods, the methods may be prohibitively expensive and/or not effective enough for many applications. Thus, the removal of 3M's fluorochemical barrier products from use has left a major void in the packaging materials market.
U.S. Pat. No. 4,521,492 discloses a method of masking the appearance of grease, fat or oil staining on a paperboard packaging substrate by first applying a non-leafing metallic ink over the substrate, followed by the application of a highly pigmented white outer layer. It is stated that the presence of the metallic ink alters the refractive index of the grease, fat or oil stained substrate and the over-coating with the highly pigmented white outer layer reduces or eliminates the metallic appearance of the lower ink layer. While effective for the masking of grease, fat or oil permeation, this method has at least two significant disadvantages. First, this material requires application of at least the non-leafing metallic ink layer using explosion-proof equipment because the non-leafing metallic ink is delivered from a solvent-based system. Second, the method requires the use of solvent recovery systems. These greatly increase the cost of utilizing this prior art grease masking system because the method cannot be used with existing mill equipment without resulting in difficulties. Further, the method of U.S. Pat. No. 4,521,492 has limited applications to printing press methods which reduces the usefulness of the process in printing processes requiring a multiple of print stations.
Against the above, it would be highly desirable to develop a method whereby the appearance of grease, fat or oil staining on a packaging material could be masked so that even if grease, fat or oil staining does occur, such stains will be less noticeable. Specifically, it would be desirable to be able to mask the appearance of grease, fat or oil staining on a packaging material whereby the materials utilized do not comprise non-leafing metallic inks Further, it would be desirable to apply such masking methods by way of aqueous systems, thereby allowing the use of standard mill equipment.